Process for Producing Steel Long Products by Continuous Casting and Rolling

ABSTRACT

In a process for producing steel long products by continuous casting of steel to form billet and bloom strands ( 5 ) and subsequent rolling of said billet and bloom strands, the liquid steel is cast in a continuous-casting permanent mould ( 3 ). The cast billet and bloom strands ( 5 ) are fed substantially without subsequent heating directly to a roll train ( 20 ). This makes the design of the plant more simple, and more cost-effective operation is made possible.

The invention relates to a process for producing steel long products according to the preamble to Claim 1 and to a continuous casting permanent mould for the process.

It is known to cut the cast strand produced by continuous casting into individual billets or blooms, to cool them to room temperature and then roll them, heated up again in a rolling mill, into steel bars, wire and other steel long products.

It is also known to place the hot (approx. 600° C.) cast billets in the rolling mill furnace for the purpose of reheating prior to subsequent rolling.

In order to save energy and to increase the output, for a long time one has been seeking solutions for how one could directly connect a continuous casting plant and a roll train, preferably a casting strand and a rolling strand.

Here the different throughputs, speeds, temperatures and temperature distributions (continuous casting surface/centre) at the outlet of the casting plant and at the inlet of the roll train are the main problems.

The minimum feed speed at the inlet of the roll train should under no circumstances come below 4.2 m/min so as to not overload the rollers thermally as a result of the long contact time with the rolling stock. Moreover, the billet at the inlet of the roll train should have a temperature suitable for rolling, both on the surface and in the core (1150-1250° C.). In particular, the surface temperature should be distributed as homogeneously as possible.

From EP-B1-0 761 327 a process is known wherein the casting strand produced in a continuous casting machine is fed to a temperature maintaining and pre-heating device, after which, in a further temperature equalisation and rapid heating device, it is heated up before being introduced into the roll train. A disadvantage here are the relatively high costs for the installation and for the operation of the reheating device.

The object forming the basis of the present invention is to propose a process which enables a simpler plant design and more cost-effective operation.

This object is achieved according to the invention by a process having the features of Claim 1.

Preferred further developments of the process according to the invention and preferred continuous casting permanent moulds for the process form the subject matter of the dependent claims.

Surprisingly, it has been established that when using continuous casting permanent moulds, as known from EP-B2-0 498 296, the billet and bloom strands produced by rapid continuous casting can be fed directly to a roll train without any additional heating. The mould cavity of these continuous casting permanent moulds has on the end of the permanent mould on the pouring in side, along a circumferential line of its cross-section, at least two circumferential sections each of which defines a cross-sectional enlargement of the mould cavity in relation to the corresponding circumferential sections of the mould cavity cross-section on the end of the permanent mould on the strand outlet side, in the form of bulges. The curvature heights of the bulges decrease in the direction of travel of the strand such that during the casting operation a strand shell forming in the mould cavity deforms upon passing through the permanent mould, and in this way even cooling and shell growth and homogeneous temperature distribution is guaranteed along the strand circumference, by means of which a high continuous casting speed allowing direct introduction into the roll train is possible. The temperature ratios in the casting strand on the outlet of the casting plant are also suitable for introduction into the roll train, and at all events temperature regulation can be provided, but no heating is required.

It is practically possible nowadays to cast strands with a tetragonal cross-section in the format range of 120-130 mm at a speed of approx. 6 m/6 min. With permanent mould cross-sectional forms according to EP-B2-0 498 296 a strand is produced with homogeneous temperature distribution on the surface, without any cold edges which would otherwise have had to have been heated inductively or by means of gas burners in front of the roll train. At the same time, with this format the technical casting advantages of a square cross-section as opposed to a round cross-section (more favourable surface/volume ratio with the same weight per meter) is largely maintained.

With one particularly preferred embodiment of a continuous casting permanent mould for the process according to the invention, the circumferential line of an approximately round mould cavity cross-section is divided on the pouring in side into at least three circumferential sections of substantially the same size, and each of these circumferential sections has on the pouring in side the cross-sectional enlargement of the mould cavity as a bulge. The curvature heights of the bulges decrease on all of the circumferential sections in the direction of travel of the strand, at least along a partial length of the mould cavity.

With a further advantageous embodiment of a continuous casting permanent mould for the process according to the invention the circumferential line of a polygonal, preferably quadrangular mould cavity cross-section has on the pouring side between all of the corners circumferential sections with cross-sectional enlargements of the mould cavity in the form of bulges, and the curvature heights of the bulges decrease on all of the circumferential sections in the direction of travel of the strand, at least of a partial length of the mould cavity.

The process according to the invention for producing steel long products, wherein the billet and bloom strands produced in the continuous casting permanent mould are fed directly—without any additional heating—to a roll train and can be rolled here into long products, enables a simple plant design and cost-effective operation.

In the following the invention will be described in greater detail by means of the drawing. This shows, purely diagrammatically:

FIG. 1 is a diagrammatic illustration of a continuous casting plant working according to the process according to the invention, having a roll train.

FIG. 1 shows diagrammatically the structure of a continuous casting plant 10 known in its own right, having a casting ladle 1, an intermediate vessel 2 (tundish), a permanent mould 3 cooled with water, and a strand deflection device 6 for the strand 5 which is drawn out by means of rollers 6′ and bent into the horizontal. After this the strand 5 is fed according to the invention directly to a roll train 20 which has a number of rolling units 21 having rolling rollers 22, not shown in any further detail because this type of roll train 20 and rolling mill are known.

A mould cavity 3′ of this permanent mould 3 has on the end of the permanent mould on the pouring in side, along a circumferential line of its cross-section, at least two circumferential sections each of which define a cross-sectional enlargement of the mould cavity 3′ in relation to the corresponding circumferential sections of the mould cavity cross-section on the end of the permanent mould 3 on the side of the strand outlet in the form of bulges, the latter not being illustrated in any greater detail.

The continuous casting speed of the strand 5 is advantageously at least 4.2 m/min so as not to exceed the admissible contact time between the strand and the rolls at the inlet of the roll train, and thus not unreasonably shortening the service life of the latter.

The temperature ratios on the outlet of the continuous casting plant 10 for introduction into the roll train are adjusted precisely so that the rolling of the strand can take place optimally. At all events, however, passive temperature equalisation can be implemented without supplying any external energy.

The mould cavity cross-section of the continuous casting permanent mould is formed substantially as a quadruple round format. This quadruple round format is based upon a square or rectangle, being provided in the corner regions, however, with relatively large radii. On the outlet side of the strand these radii are in the corner regions advantageously approximately 20 to 40 mm with a length or width of the mould cavity cross-section of 120 to 180 mm.

For example, with a 120 mm quadruple round format an edge rounding of approx. 25 mm and the continuous casting speed of min. 5.2 m/min, and correspondingly with a 180 mm quadruple round format an edge rounding of approx. 40 mm and casting speed of at least 4.2 m/min could be provided.

At the discharge 11 of the intermediate vessel 2 located above the permanent mould 3 there is a conventional regulating device, preferably a slide closure 15 for the purpose of precise inflow regulation of the molten steel to be cast into the permanent mould. Regulation of the filling level height of the molten mass in the permanent mould 3 is implemented dependently upon the inlet speed in the first rolling stand (so-called master/slave operation), for which a regulating device 13 and a filling level measuring device 14 are provided on the permanent mould 3. With this regulation an approximately constant filling height should be achieved in the permanent mould. Moreover, a fireproof immersion pipe attached to the inflow regulating device and projecting into the permanent mould is advantageously provided so that the smoothest possible bath surface is produced in the permanent mould.

The cast billet strands form a substantially quadruple round format, and the rounding of the latter corresponding to the format is chosen such that with a sufficiently high casting speed a strand with temperature distribution required for the rolling is produced which can be directly rolled without any active subsequent heating, i.e. the cast strands are fed infinitely to the roll train without being divided into pieces. 

1. A process for producing steel long products by continuous casting of steel to form billet and bloom strands and subsequent rolling of these billet and bloom strands to form the long products, such as steel bars or wire, whereby the liquid steel being cast into a continuous casting permanent mould, characterised in that the cast billet respective bloom strands are fed directly to a roll train substantially without any subsequent heating.
 2. The process according to claim 1, characterised in that the cast billet strands form a substantially quadruple round format.
 3. The process according to claim 2, characterised in that the rounding with the quadruple round format is chosen corresponding to the format such that with a sufficiently high casting speed a strand with temperature distribution required for rolling is produced which can be rolled directly without any active subsequent heating.
 4. The process according to claim 1, characterised in that the cast billet strands are substantially circular in form.
 5. The process according to claim 1, characterised in that the continuous casting permanent mould forms a mould cavity open on both sides, the mould cavity having on the end of the continuous moulding permanent mould on the pouring in side, along a circumferential line of its cross-section, at least two circumferential sections which each define a cross-sectional enlargement of the mould cavity in relation to the corresponding circumferential sections of the mould cavity cross-section on the end of the permanent mould on the strand outlet side, in the form of bulges, and curvature heights of the bulges decrease in the direction of travel of the strand such that during the casting operation a strand shell forming in the mould cavity deforms upon passing through the permanent mould and in this way guarantees rapid and even cooling and homogeneous temperature distribution along the strand circumference.
 6. The process according to claim 1, characterised in that the continuous casting speed is at least 4.2 m/min.
 7. The process according to claim 1, characterised in that at the discharge of the intermediate vessel located above the permanent mould an adjustment device, preferably a slide closure, is provided for the purpose of precise regulation of the molten steel to be cast into the permanent mould, regulation of the filling level of the molten mass in the permanent mould being implemented dependently upon the inlet speed into the first roll stand of the roll train, and the regulating device being correspondingly regulated.
 8. The process according to claim 1, characterised in that commercial grade and concrete steel are used.
 9. The process according to claim 1, characterised in that quality steel is used.
 10. A continuous casting permanent mould for the process according to claim 1, characterised in that the circumferential line of a polygonal, preferably quadrangular mould cavity cross-section has on the pouring in side between all of the corners circumferential sections with cross-sectional enlargements of the mould cavity in the form of bulges, and the curvature heights of the bulges decrease on all of the circumferential sections in the direction of travel of the strand, at least along a partial length of the mould cavity.
 11. The continuous casting permanent mould according to claim 10, characterised in that the mould cavity cross-section of the continuous casting permanent mould substantially forms a quadruple round format which is quadratic or rectangular in form, but is provided in the corner regions with radii.
 12. The continuous casting permanent mould according to claim 11, characterised in that with the continuous casting permanent mould, on the strand outlet side, the radii in the corner regions are approximately 20 to 40 mm with a thickness or width of the mould cavity cross-section of 120 to 180 mm. 